Crystallographical Characterization of Initiation of Intergranular Stress Corrosion Cracking of Alloy 600 in PWR Environment

Abstract

Susceptibility of intergranular stress corrosion cracking of Alloy 600 in simulated primary water environment of pressurized water reactor with dissolved hydrogen of 2.75 ppm at 360°C was evaluated by means of slow strain rate testing (SSRT) and numerical analysis to predict the microscopic stress distribution. Mill annealed and 10 or 20 % cold rolled Alloy 600 flat tensile specimens were mechanically polished using SiC papers, then finished with colloidal silica. SSRT was terminated at the tensile strain of 10 %. Many IGSCC cracks were observed on the surface of the tensile specimen. The surface of tensile specimens were characterized by EBSD to investigate the crystallographical feature of the intergranular crack initiation. Alloy 600 tends to exhibit IGSCC at around a 30 degree of misorientation angle. Most crack initiate at random grain boundaries. On the other hand, coincidence boundaries, such as Σ3, Σ9, exhibit no cracks. Susceptibility to IGSCC was also evaluated by numerical simulation in which the crystalline structure of actual test specimen was FEM modeled. In the numerical analysis microscopic stress distribution in each crystals revealed the preferential grain boundaries of IGSCC. The numerical analysis is in fair agreement with the experimental results. Therefore, the numerical calculation proposed is an important method to predict SCC and also the combination of numerical analysis and SSRT with EBSD observation provides basic insight on the mechanism of SCC in high temperature and high pressure water environment.